1. Field of the Invention
The present invention relates to pattern matching in image processing technology.
2. Description of the Related Art
In recent years, a practical method for detecting a target pattern from an image by image processing has been proposed. In particular, if a target pattern is a human face, various applications can be considered. Research and development into detection methods is active. Furthermore, research and development is increasingly being made on a face recognition method for not only detecting a face but also identifying which of a set of preregistered people matches the detected face.
A matching method is often used as a detection/recognition method. In the matching method, a similarity between a template prepared in advance (registration image) and a processing image is calculated by recognition processing such as normalized correlation.
If the face detection/recognition technique using such as the matching method is applied in a real environment such as a monitoring system, the contrast of an image may change or a partial shadow may be produced in an image in accordance with weather changes or the time of day. In the real environment, variations in illumination condition may change an image appearance (pattern), and also largely influence detection/recognition accuracy.
To reduce this influence, instead of executing recognition processing for the luminance value of an image, it is proposed to extract a feature amount robust against an illumination variation from the luminance value, and then execute recognition processing for the extracted feature amount. Japanese Patent No. 3831232 describes, for example, a method for performing correlation processing for signs of luminance differences (peripheral increment signs) between a target pixel and its surrounding pixels. In this method, the increments (magnitude relationships) between the density value of the target pixel and those of the surrounding pixels are represented by signs in each of a template image and a processing image, and the number of matching signs is set as a similarity. It is known that such methods make it possible to execute matching which is robust against noise mixing and luminance variation within a range in which a sign is not inverted.
The technique described in Japanese Patent No. 3831232, however, calculates the similarity between the template image and the processing image. It was difficult to actually extend detection targets to those belonging to the same category but having their own individualities such as faces because it was necessary to prepare templates, the number of which is equal to that of faces.
To solve the problem, in Japanese Patent No. 3696212 and Japanese Patent Laid-Open No. 2006-146626, for example, the statistical characteristics of peripheral increment signs are extracted from a database containing detection target images such as various face images.
To detect a target pattern in a conventional manner, however, a large amount of memory was necessary to implement a table to hold measured occurrence probabilities using a database.
Applying a conventional method to a face recognition method which identifies, based on a face, which of a set of preregistered people matches the detected one makes a table size increase proportionately with the number of registered people.